1. Field of the Invention
The present invention relates to an electroacoustic converter which is used for a thin piezoelectric speaker, microphone, or the like using a piezoelectric film as a vibrating body.
2. Description of the Related Art
A so-called piezoelectric film in which electrode layers are formed on both surfaces of a sheet-like piezoelectric material, such as a polymeric piezoelectric material, for example, a uniaxially stretched polyvinylidene fluoride (PVDF) film, or a polymeric composite piezoelectric body in which a piezoelectric material in a powder form is dispersed in a polymer material as a matrix has the property of expanding and contracting in response to an applied voltage. In order to use this piezoelectric film as a speaker, it is necessary to convert expansion and contraction movement along the film surface to vibration in a direction perpendicular to the film surface. The conversion from expansion and contraction movement to vibration is achieved by maintaining the piezoelectric film in a curved state, and with this, it is possible to make the piezoelectric film function as a speaker.
However, in general, the piezoelectric film itself has low stiffness. For this reason, if the area of the speaker becomes larger, the piezoelectric film is bent due to the weight thereof, and it is difficult to maintain the piezoelectric film in the curved state. For this reason, there is a limit to the increase in size of a speaker using a piezoelectric film.
In regard to this problem, a mechanical bias is applied to the piezoelectric film. For example, JP1978-59473A (JP-S53-59473A) describes an electroacoustic converter (portable sound generation device) in which thin-film electrodes are formed on both surfaces of a polymeric piezoelectric material sheet by vapor deposition or the like, one end of the thin-film electrode is fixed to a case through a backing, and the other thin-film electrode is brought into press-contact with a conductive film formed on a member applying a mechanical bias.
JP1978-59473A (JP-S53-59473A) describes a member having a loose curvature as a member applying a mechanical bias for use in an electroacoustic converter.
Specifically, a configuration is described in which both ends of the piezoelectric film in the expansion and contraction direction are fixed to a mounting plate with gaps therebetween on both sides substantially parallel to the expansion and contraction direction, and the thin-film electrode on the side of the piezoelectric film opposite to a sonic wave radiation direction is pressed to a member having a curvature through a ground plate, thereby providing electrical conduction between the member having a loose curvature and the ground plate.
In the electroacoustic converter described in JP1978-59473A (JP-S53-59473A), the piezoelectric film with the periphery fixed has a curved shape in which the piezoelectric film is pressed to the member having a loose curvature for applying a mechanical bias.
In this electroacoustic converter, the member applying a mechanical bias has a loose curvature, whereby it is possible to apply a constant mechanical bias at any location of the piezoelectric film. For this reason, the expansion and contraction movement of the piezoelectric film is converted to forth-back movement without waste, and sound corresponding to supplied power is generated.
In JP1978-59473A (JP-S53-59473A), the electroacoustic converter has such a configuration, whereby it is possible to freely select a timbre over a wide frequency band and to achieve reduction in the number of parts, simplification of a configuration or a reliable mechanism, or the like.
However, if the piezoelectric film has a curvature, since the piezoelectric film is curved, there is a restriction on an installation place or a mounting method, and it is not suitable for the purposes of wall-mounting or installing on the rear surface of a picture, a poster, a decoration plate, or the like. If the area of the speaker becomes larger, the thickness of the piezoelectric file becomes larger even if the piezoelectric film has a loose curvature, and the original feature as the thin speaker is degraded.
In order to compensate for such a problem, the curvature of the piezoelectric film may be made small. That is, the radius of curvature may be made large. However, if the piezoelectric film is made nearly planar, it is not possible to convert the expansion and contraction movement of the piezoelectric film to the forth-back movement, sound is not emitted, and a sound pressure (sound volume) is made small.
In contrast, the inventors have suggested an electroacoustic converter having a piezoelectric film which contracts with application of a drive voltage, a viscoelastic support which is in close contact with one surface of the piezoelectric film, and a pressing member which presses a peripheral portion of the piezoelectric film to press the piezoelectric film to the viscoelastic support, the piezoelectric film having a flat portion which is supported linearly by the surface of the viscoelastic support and an inclined portion which is formed in the outer circumference of the flat portion and is inclined from the flat portion toward a pressing position by the pressing member (JP2014-17799A).